Recently, interest in energy storage technology is increasing day by day. As the application field is extended to mobile phones, camcorders, and laptop computers, and further to energy of electric vehicles, the demand for higher energy density of batteries used as a source of power of electronic devices is growing. Lithium secondary batteries that best satisfy the demand are now being actively studied.
In currently available secondary batteries, lithium secondary batteries developed in the early 1990's are composed of an anode made of carbon material capable of intercalation or deintercalation of lithium ions, a cathode made of lithium-containing oxide, and a non-aqueous electrolyte solution in which an optimal amount of lithium salt is dissolved in a mixed organic solvent.
An average discharge voltage of lithium secondary batteries is about 3.6 to 3.7V that is higher than the discharge voltage of other batteries such as alkali batteries or nickel-cadmium batteries, and this is one of advantages of lithium secondary batteries. For such high operating voltage, the composition of electrolyte solutions that is electrochemically stable in the charge/discharge voltage range of 0 to 4.2V is required. To this end, the electrolyte solution comprises a mixed solvent of cyclic carbonate compounds such as ethylene carbonate and propylene carbonate, and linear carbonate compounds such as dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate as a solvent. As a solute of the electrolyte solutes, lithium salts such as LiPF6, LiBF4 and LiClO4 may be used, and they acts as a source of lithium ions in batteries to allow the lithium batteries to operate.
In the initial charging process of lithium secondary batteries, lithium ions released from a cathode active material, for example, lithium metal oxide, move to an anode active material, for example, graphite, and intercalate into a layer of the anode active material. In this instance, lithium which is highly reactive, reacts with the electrolyte solution and carbon that is a component of the anode active material, graphite, on the surface of the anode active material, yielding a compound such as Li2CO3, Li2O and LiOH. These compounds form a sort of solid electrolyte interface (SEI) layer on the surface of the anode active material, graphite.
The SEI layer serves as an ion tunnel, allowing only lithium ions to pass through. By virtue of the ion tunnel effect, the SEI layer prevents the organic solvent from intercalation into the layers of the anode active material and from destroying the structure of the anode, the organic solvent having molecules of high molecular weight in the electrolyte solution and moving with lithium ions therein. Accordingly, the contact of the electrolyte solution with the anode active material is avoided, the decomposition of the electrolyte solution does not take place, and the amount of lithium ions in the electrolyte solution is reversibly maintained, allowing stable charging/discharging.
However, in the SEI layer forming reaction, gas such as CO, CO2, CH4 and C2H6 is generated by decomposition of carbonate-based solvents, causing batteries to swell and increase in thickness during charging. If fully charged batteries are kept at high temperature, the SEI layer is slowly destroyed over time due to increased electrochemical energy and thermal energy, and side reactions continue to occur between the exposed surface of the anode and the surrounding electrolyte solution. The gas that is continuously generated increases the internal pressure of batteries, and as a result, the batteries increase in thickness, causing damage to electronic products, such as mobile phones and laptop computers, to which the batteries are applied. That is, safety at high temperature is poor. Furthermore, in common lithium secondary batteries including a large amount of ethylene carbonate, the SEI layer is unstable, leading to severe internal pressure increase of batteries. Moreover, ethylene carbonate has a high freezing point of 37 to 39° C. and is in solid state at room temperature, in turn, has low ionic conductivity at low temperature, and accordingly, lithium batteries using non-aqueous solvents containing a large amount of ethylene carbonate have poor conductivity at low temperature.
To solve the problem, attempts have been made to modify the SEI layer forming reaction by variously changing the ingredient composition of carbonate organic solvents or adding certain additives. However, so far as is known, the change of solvent ingredients or the addition of certain compounds to electrolyte solutions for the purpose of improving the battery performance may improve some performance characteristics, while degrading other performance characteristics.
Accordingly, there is an urgent need for development of the composition of non-aqueous electrolyte solutions for providing lithium secondary batteries that are superior in terms of high rate charging/discharging characteristics, cycle life, and discharging characteristics at low and high temperature.